As an apparatus for measuring the resistivity of IPA (isopropyl alcohol), which is insulating liquid, one disclosed in Patent Literature 1 has been contrived. The resistivity-measuring apparatus is one that measures the resistivity of IPA positioned between an outer electrode and an inner electrode by applying AC voltage between the outer electrode and the inner electrode.
In Patent Literature 1, the resistivity of IPA is approximately 1000 MΩ·cm, and as the AC voltage applied between the outer electrode and the inner electrode, AC voltage having an amplitude of approximately 2 V and a frequency of 100 Hz is used. Note that the resistivity of pure water or the like in addition to IPA is also generally measured using similar AC voltage.
On the other hand, to measure the resistivity of oil such as lubricating oil in order to sense the deterioration of the oil, using the above-described resistivity-measuring apparatus is considered.
However, since the resistivity of oil is several tens times to several hundred times higher as compared with the resistivity of IPA, current flowing through a measuring circuit having the outer electrode and the inner electrode is small, making the measurement difficult, and also floating capacitance of the measuring circuit deteriorates AC voltage responsiveness. As a result, in the case of applying the AC voltage having a frequency of 100 Hz, before a signal (e.g., output voltage) outputted from the measuring circuit stabilizes, the polarity of the AC voltage is switched, thus giving rise to the problem of being unable to accurately measure the resistivity of oil.
Also, a possible way to measure the resistivity of oil is to set voltage applied between the outer electrode and the inner electrode to a high voltage such as 1000 V to increase detected current. Note that to measure the resistivity of oil, high voltage has been generally applied in the past.
However, in the case of applying a high voltage of, for example, 1000 V to oil, the oil is oxidized and changed in quality, i.e., there is the problem that the measurement causes the deterioration of the oil.